Of course , we think more. In 20 KV system we don’t have zero phase sequence current as any result of unbalace load . Any unbalance load appears as negative phase sequence , and NPS current doesn’t pass zigzag earthing transformer. This is the question , How 100 KVA or 8.67 Amp. is achieved ?If neutral point of zigzag earthing transformer is grounded solidly , so the earth fault current won’t be 8.67 Amp. It will be much higher. and if it has a secondary wye winding the earth fault depends on probable resistance (if any) and impedance voltage . the earthing scheme of your idea isn’t clear . It’s not clear why the continuous rating is 100 KVA.